It is because the mobile phone system restricts SMS notifications. The solution is as follows:

1. First, open the settings indicated by the arrow on the mobile phone desktop.

2. After entering the settings page, click the notification and status bar indicated by the arrow.

3. After entering the notification and management status bar interface, click notification management at the top of the interface.

4. On the page that appears, click information.

5. Next, click the arrow to allow notifications.